sorry for this omission, please assume sw2 and sw3 are connected as well.All i want to know about dummy frames that a switch configured with uplink feature, send at multicast address 0100.0ccd.cdcd, using mac address from the mac table as source address.Will switch send dummy frames for all mac addresses present in mac table? or

will it send only dummy frames for only those mac addresses belonging to hosts which are directly connected to switch?

thanks a lot!

Dummy frames - no dummy frames in STP and/or uplink fast.

Read the below link:-

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080094641.shtml

HTH>

thanks for the link Andrew!

But my question remain unresolved.For example in the link you forwarded,after uplink feature brings the blocked port up, swA sends dummy packet for host C. Will swA also send dummy frame for host s which is connected to swD2?

If yes, then it will cause swD2 to learn errornously host S is reachable via port connected to swA, while in fact host S is directly connected to swD2.

thanks a lot!

Dummy frames? what do you mean?

Spanningtree does not use dummy frames.

thanks for your reply

i am talking about uplink feature that use dummy frames. Please check the link you forwarded to me.

OK - now I understand what you are refering to, it's is the update of CAM tables. I mis-understood the "dummy" wording, when actually they are carefully crafted BPDU's send by the switch to force the rest of the switches to update the CAM tables - when there have been a change in the topology - which can take some time, as the TCN bit is set by the switch that has the link failure - and NO other switch in the layer 2 topology will "timeout there MAC address tables" until they recevie a BPDU from the root switch with the TCN ACK bit set - which then triggers the switches that recevie it to re-learn the mac address table.

The page quotes:-

"The backup link is brought up so quickly, however, that the CAM tables are no longer accurate. If S sends a packet to C, it is forwarded to D1, where it is dropped. Communication between S and C is interrupted as long as the CAM table is incorrect. Even with the topology change mechanism, it can take up to 15 seconds before the problem is solved.

In order to solve this problem, switch A begins to flood dummy packets with the different MAC addresses that it has in its CAM table as a source. In this case, a packet with C as a source address is generated by A. Its destination is a Cisco proprietary multicast MAC address that ensures that the packet is flooded on the whole network and updates the necessary CAM tables on the other switches. "

A clearer undertstanding of the TCN and CAm table update is available in basic STP explanations.

HTH>

thanks for your reply Andrew! I applogise for the confusion. I just used the word dummy because that is what cisco book bscmsn guide refer to them.Anyway in the last paragrah you said" In order to solve this problem, swA begins to flood dummy packets with different Mac addresses that it has in its Cam table as a source. In this case , a packet with C as source address is generated by A"

My question is since swA cam table has entry for host S which is directly connected to swD2, would switchA send dummy packet using S mac address as a soouce mac address?

If yes, then swD2, which previouldy knew that s is directly connected to its port,will relearn that host S is now reachable by port connected to swA.

thanks a lot!